As the capacity of a hard disk drive (HDD) increases in recent years, attention is being paid to an MR head using an element whose electric resistance changes according to fluctuations in external magnetic fields. Particularly, the sensitivity of a giant magnetoresistive (GMR) head and a tunnel magnetoresistive (TMR) head is very high, so that recording density of a magnetic disk can be increased. Further, as recording density becomes higher, miniaturization of an MR element is being promoted.
An MR head has a magnetoresistive element (MR element) whose two side surfaces are surrounded by magnetic layers which apply a bias magnetic field. When the MR element is miniaturized, naturally, the space which can be used by the bias magnetic layer is also regulated. When the volume of the magnetic layer and the areas of two side surfaces of the MR element are reduced, the bias magnetic field decreases.
The areas of the two side surfaces of the MR element are determined by a read gap (distance between two shields surrounding a magnetic tunnel junction (MTJ) or GMR stack layer) and stripe height (horizontal (depth) dimension of the MR element forming a right angle with the surface of a recording medium). Decrease in the read gap width is necessary to increase linear resolution (on tracks), and decrease in stripe height causes decrease in the width of the read head necessary to decrease the sensitivity to a track edge.
A typical sensor structure includes an antiferromagnetic (AFM) pinning layer, a synthetic antiferromagnetic layer (SAF), a nonmagnetic spacer or a tunnel insulator, and a ferromagnetic free layer. A seed layer and a capping layer are also used for various purposes. The SAF is made of two ferromagnetic members coupled in opposite directions via a thin spacer layer. The ferromagnetic member in the SAF includes a pinned layer which is in contact with the AFM layer and a reference layer which is in contact with the nonmagnetic spacer layer or the tunnel insulator. A resistance change via the reader stack is determined by relative directions of magnetizations between the reference layer and the free layer. In the free layer, the magnetic field is biased and oriented to form a right angle with the reference layer. With the configuration, reading sensitivity becomes very high, and a linear response can be obtained to an external magnetic field from a recording medium. The bias magnetic field is also called “hard bias” and is expected to be maintained constant throughout the life of a disk drive. The hard bias has a role of preventing creation of a magnetic domain in a free layer. Both the sensor and the hard bias are sandwiched by two thick soft magnetic shields.
A simple hard bias stack body includes an underlayer made of Cr, W, or the like, a magnetic layer made of CoPt or CoCrPt, and a capping layer made of Cr, Ru, or Ta. To prevent switching caused by an external magnetic field at particularly high operation temperature, the coercive force (Hc) of the magnetic layer is desired to be equal to or higher than 159.5 kA/m (2000 oersted (Oe)).
When magnetization reversal occurs in a part of magnetic layer crystal grains, there is the possibility that remarkable decrease in the bias magnetic field is caused, and noise in a sensor is induced. Reduction in the read gap size leads to decrease in thickness of the hard bias stack body which can be applied between shields. Since the bias magnetic field is proportional to the product (Mrt) between residual magnetization of the magnetic layer and thickness, when the thickness “t” decreases, application of bias to the free layer may become insufficient. Further, when the magnetic layer and the shield layer become close to each other, a leakage magnetic flux to the shield layer increases, and the bias magnetic field in the junction wall surface (the border between the reader stack and the hard bias stack body) further decreases.
One of methods of increasing the magnetic field is to decrease the thickness of the insulating layer that insulates the magnetic layer from the free layer in the junction wall surface. However, since a low leak current and a high breakdown voltage are requested, there is a limit to decrease the thickness of the insulator. The magnetic layer can be made of an insulating material such as ferrite. By making the magnetic layer of an insulating material, the insulating layer may not be provided, or the thickness of the insulating layer can be decreased to 3 nm or less. However, there is a tendency that saturated magnetizations and coercive forces of most of insulating magnetic ferrites are inferior to those of Co—Pt alloys. It is much difficult to control the compositions and crystal growth of the ferrites.
The present CoPt-based hard bias stack body has two-dimensional isotropy. In a plane, the coercive forces Hc along any directions are equal. That is, OR (orientation ratio, that is, the ratio between coercive force in an in-plane perpendicular direction with respect to the stripe height and coercive force in the stripe height direction) indicative of magnitude of magnetic anisotropy is equal to 1. Hexagonal crystal c-axes of CoPt are at random in a plane. However, by exchange coupling of a number of crystal grains, a relatively high squareness ratio (0.85 or higher) can be realized. On the junction wall surface, an average magnetic field is directed toward the free layer. When the stripe height decreases, the crystal grains in the junction wall surface decrease, so that it becomes more difficult to direct the magnetic flux toward the free layer. This phenomenon is conspicuous when the c-axes of the crystal grains are not oriented to the free layer. If the c-axes can be oriented toward the junction wall surface, the ratio of the stripe height (depth) to the crystal grain diameter is not a matter. Further, Mr to the same thickness “t” increases, and a higher bias magnetic field can be obtained. A larger number of magnetic fluxes are condensed on the junction wall surface, and the magnetic fluxes which are lost at side ends of the hard bias stack body decrease.
A Cr seed layer is grown in a (110) lattice plane. From the studies of OR in longitudinal media, OR>1 is achieved only in the case of a Cr (002) lattice plane. A CoPt (1120) is formed on it. With respect to the epitaxial relations between the [110] direction and [1-10] direction, for CoPt (in the (1120) lattice plane, the lattice constant in the c-axis direction is 0.41 nm, and that of a lattice axis perpendicular to the c-axis is 0.43 nm), it is equivalent in energy. Only in the case where a Cr lattice is deformed in a plane due to an anisotropic stress, a specific direction is desired. Simions et al. (refer to patent document 1) propose different seed layers made of MgO, NiAl, and the like. In study of recording media, it was proven that both underlayers provide two-dimensional c-axis alignment.
However, Larson et al. (refer to patent document 2) and San Ho et al. (refer to patent document 3) disclose that in-plane anisotropy can be realized by formation of a film of CoPt alloy using oblique sputtering.
In-plane anisotropy of a soft layer of FeCo or the like can be easily realized by oblique sputtering. Particularly, in a sputtering process having a high incidence angle to normal of a film formation face, in-plane anisotropy occurs even in a relatively thin film (about 10 nm) by the self shadow effect. The self shadow effect denotes that a shadow is created by nucleus generated on the surface of an oblique incidence deposition film and, since sputter particles do not fly in the shadow portion, the film grows in an oblique column shape. In our experience, in a CoPt layer having an optimum thickness (about 20 nm), dependency of in-plane anisotropy on the incident angle is low, so that a seed layer or an underlayer has to be thickened. However, a seed layer has to be thin (6 nm or less), and it makes it very difficult to form a hard bias stack film according to a result of study of Larson et al. and San Ho et al. San Ho et al. suggests that a magnetic layer has a (11-20) lattice plane to show OR of a certain degree. In evaluation by an XRD (X-ray diffractiometer), a (10-10) lattice plane is shown. An obliquely deposited underlayer does not display the (002) plane which is considered to be necessary to induce the OR in a longitudinal recording medium (Mirzamaani). As suggested by the concept of Larson et al., the hard bias OR is induced by probably anisotropy caused by exchange coupling. “Mrt” is the largest along a direction in which the exchange coupling is the maximum. It is considered that OR is induced by a wavy surface pattern (anisotropy roughness by Carey et al. (refer to patent document 4)).
The present hard bias deposition is performed mostly by the long throw sputtering such as ion beam deposition (IBD). An IBD system has a stage which is rotatable to adjust the incidence angle of an incident sputter particle. For example, Hegde et al. (refer to patent document 5) disclose methods of depositing hard bias films. A magnetic layer is deposited at an almost perpendicular angle (25 degrees or less from the perpendicular line).